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91

Information Notice No. 93­91: Misadjustment Between General Electric

4.16­KV Circuit Breakers and their Associated Cubicles

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

December 3, 1993

NRC INFORMATION NOTICE 93‐91: MISADJUSTMENT BETWEEN GENERAL ELECTRIC 4.16‐KV

CIRCUIT BREAKERS AND THEIR ASSOCIATED CUBICLES

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees to the potential for multiple common‐cause failures

of logic functions due to misadjustment between General Electric (GE) 4.16‐kv

Magne‐Blast circuit breakers and their associated cubicles. It is expected

that recipients will review the information for applicability to their

facilities and consider actions, as appropriate, to avoid similar problems.

However, suggestions contained in this information notice are not NRC

requirements; therefore, no specific action or written response is required.

Description of Circumstances

During surveillance testing at the Duane Arnold Energy Center on

September 16, 1993, the "B" standby diesel generator output breaker (1A411) to

essential bus 1A4 failed to close after simulation of a loss of offsite power

(LOOP) and loss of coolant accident (LOCA) signal. During normal plant

operation, the normally closed feeder breaker (1A402) from the startup

transformer and the normally open feeder breaker (1A401) from the standby

transformer provide essential bus 1A4 with two offsite sources of power. One

of the "b" contacts (closed when the breaker is open) from each offsite power

feeder breaker stationary auxiliary switch (mounted in the breaker cubicle) is

used in the diesel generator output breaker automatic close‐permissive circuit

to indicate whether the two offsite power feeder breakers are open or closed.

A closed indication from either breaker 1A401 or 1A402 will prevent the diesel

generator output breaker from closing automatically (unsynchronized) onto

essential bus 1A4. Breakers 1A401, 1A402, and 1A411 are GE Magne‐Blast types.

Troubleshooting revealed that the auto‐closing logic for the diesel generator

output breaker was not being completed because a "b" contact from breaker

1A401 was open, indicating that the breaker was closed when the breaker was

actually open. When the breaker 1A401 is open and in its fully elevated

operating position in the cubicle, there is supposed to be a small gap or
clearance between the breaker plunger and the stationary auxiliary switch

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IN 93‐91

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operating rod (see Attachment 1, Figure 2) to ensure positive closure of all

"b" contacts in the stationary auxiliary switch. However, the plunger of

breaker 1A401 was found to be actually pushing its stationary auxiliary switch

operating rod up about 4.8 mm [3/16 inch] with the breaker open. This

apparently occurred when the breaker was last elevated into its operating

position which then opened one or more "b" contacts (including the one in the

diesel generator breaker control circuit) from their normally closed position.

The misalignment rendered the "B" diesel generator inoperable from

July 21, 1993, to September 25, 1993, because it was not capable of

automatically closing onto the 1A4 bus.

Upon investigation, the licensee determined that it had performed routine

preventive maintenance on breaker 1A401 (a vertically racking, GE model

AM‐4.16‐350‐2H) on July 21, 1993, which consisted essentially of lowering the

breaker to the bottom of the cubicle, inspecting and lubricating it, and

cycling the breaker after elevating it to its test position. However, cycling

the breaker in the test position does not operate the stationary auxiliary

switch unless a special test link is fitted. Therefore, this action alone

does not verify the various control functions performed by the stationary

auxiliary switch contacts. The "B" diesel generator monthly tests were

completed satisfactorily during the operating cycle, but these tests use a

synchronized manual closure of the diesel generator output breaker, which

bypasses the breaker automatic close permissive (dead bus) circuit. Even

though the once‐per‐operating‐cycle integrated LOOP/LOCA test detected the

failure, the potential existed for the "B" diesel generator to be inoperable

for an entire operating cycle, or when required during an outage. The

periodic routine preventive maintenance performed on breaker 1A401, which may

have resulted in the breaker misadjustment and opening of a stationary

auxiliary switch contact, was not scheduled in conjunction with a surveillance

test (such as the integrated LOOP/LOCA test) that would verify proper

operation of the auxiliary switch contacts.

Discussion

Misadjustment between the stationary auxiliary switch and the breaker plunger

on 4.16‐kv breakers has the potential to cause multiple common‐cause failures

of many different logic functions that rely on these auxiliary switch

contacts. The circuit breaker plunger to auxiliary switch operating rod

adjustment may be affected by breaker maintenance, switchgear maintenance,

differences in operating plunger height of interchangeable breakers,

differences in breaker cubicles and breaker position switches, and non‐

repeatability in breaker height when racking in breakers.

During breaker maintenance on July 21, 1993, the licensee for the Duane Arnold

plant checked and adjusted the height between the breaker lifting rail and the

operating plunger in accordance with the applicable GE breaker technical

manual (GEK‐7320F, Revision 2/80, "Magne‐Blast Circuit Breakers Types AM‐4.16‐

350‐2C and ‐2H"). However, the licensee did not verify the open breaker

plunger‐to‐operating‐rod clearance of 0 to 3.2 mm [0 to 1/8 inch] as

recommended in the applicable GE switchgear technical manual (GEH‐1802,

"Instructions ‐ Metal‐Clad Switchgear Types M26 and M36 for Magne‐Blast

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IN 93‐91

December 3, 1993
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Circuit Breakers Types AM‐4.16 and AM‐13.8"). In order to ensure a minimum

gap for positive "b" contact closure, the manufacturer (now called the GE

Specialty Breaker product department) provided additional guidance in its

Service Advice Letter (SAL) 073‐323.1, issued February 1, 1978, specifying a

gap of 1.6 mm [1/16 inch] to 3.2 mm [1/8 inch]. This SAL also provides the

recommended sequence of adjustments.

Further conversations between the licensee and GE determined that there are

additional dimensional checks (not contained in either the breaker or

switchgear technical manual) which would provide additional assurance of

proper auxiliary contact operation after breaker maintenance. These

additional dimensional checks for a closed breaker (see Attachment 1,

Figure 1) are:

� 7.9 � 1.6 mm [5/16 � 1/16 inch] from the top of the circuit breaker plunger

nut to the bottom of the auxiliary switch operating rod casing.

� All "a" contacts are closed with at least 23.8 mm [15/16 inch] travel of the

operating rod.

NOTE: Total specified travel of the plunger is 27 � 1.6 mm

[1 1/16 � 1/16 inch]. If the total plunger travel is set at the minimum

specified value of 25.4 mm [1 inch] and the plunger‐to‐operating‐rod gap

is set at the maximum specified value of 3.2 mm [1/8 inch], the amount

of travel of the operating rod would only be 22.2 mm [7/8 inch], or less

than the required 23.8 mm [15/16 inch] necessary to fully ensure closure

of the "a" auxiliary contacts when the breaker closes.

For an open breaker (see Attachment 1, Figure 2) the additional dimensional

check is:

� The operating rod collar should rest on the operating rod casing, and the

operating rod should not be displaced by the plunger of an open breaker.

GE stated that it will issue a SAL to clarify the instructions and dimensions

contained in the previous publications. Meanwhile, licensee inquiries on this

subject may be directed to GE Nuclear Energy Power Delivery Services in

King of Prussia, Pennsylvania (Phone: 215‐992‐6049/Fax: 215‐992‐6191.)

Related Generic Communications

� NRC Information Notice 91‐55, "Failures Caused by an Improperly Adjusted

Test Link in 4.16 KV General Electric Switchgear," dated

September 16, 1991.

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No specific action or written response is required by this information notice.

If you have any questions regarding this matter, please contact one of the

technical contacts listed below or the appropriate Office of Nuclear Reactor

Regulation (NRR) project manager.

/S/'D BY BKGRIMES

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation
Technical contacts: Jay A. Hopkins, RIII

(319) 851‐5111

David S. Butler, RIII

(708) 790‐5796

Stephen D. Alexander, NRR

(301) 504‐2995

Attachments:

1. Magne‐Blast Circuit Breaker Plunger and

Stationary Auxiliary Switch Adjustments

2. List of Recently Issued NRC Information Notices

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